Ascites cells of the thymic lymphoma BALUNLM 1798 are sensitive to glucocorticoids. When inoculated subcutaneously, these cells grow to yield glucocorticoid resistant tumors. We propose that this is a developmental transition dependent not upon mutation but rather upon differentiation. Genetic, biochemical, and cytological experiments are described which are designed to test this hypothesis and to investigate the mechanism(s) which governs this putative developmental transition from a glucocorticoid sensitive.